Shape memory materials are materials that can change their physical conformation when exposed to an external stimulus, such as a change in temperature. Such materials have a permanent shape, but can be reshaped above a critical temperature and fixed into a temporary shape when cooled under stress to below the critical temperature. When reheated above the critical temperature (“Tc”, also sometimes called the triggering temperature), the material reverts to the permanent shape. Certain polymers can have shape memory properties.
Shape memory is an inherent property of certain polymers that can arise, in part, from rubber elasticity. One example of rubber elasticity occurs when a crosslinked rubber is stretched and deformed several hundred percent, it still retains the memory of its original shape, and will return to that original shape when the external stress is released. The origin of this well-known phenomenon is changes in the conformational entropy of the network chains. This is distinct from the phenomenon of shape memory, which arises when the elastomer is deformed above a critical temperature, Tc, frozen into a temporary shape that is stable below Tc, and then heated again above Tc to recover the original shape. To accomplish this, a second “temporary” or reversible network needs to be formed below Tc, but disappear above Tc.
Thus, at least two crosslinked networks are present in the microstructure of shape memory polymers. A primary network provides permanent crosslinks and the permanent shape of the material. This network is usually composed of covalent bonds, but it may rely on physical bonds (e.g., crystallites, hydrogen bonding, ionic interactions, vitrification, or nanophase separation) if the relaxation times of these “bonds” are sufficiently long such that the bonds behave mechanically as permanent within the timeframe of the use of the material. A second network relies on labile physical bonds, as opposed to covalent bonds, to allow for thermal reversibility of the network. The secondary network is reversible at Tc, so that for a temperature greater than Tc, the network diminishes or disappears, and the material can be deformed to a new shape. When the material is cooled to below Tc, while maintaining the deformation, the physical network reforms into the temporary shape of the material. When reheated above Tc in the absence of external stress, the original shape of the material, that is, the permanent shape is recovered.
In most known shape memory polymers, shape memory is provided by the polymer structure itself, although many applications include fillers and additives to adjust the modulus and/or strength of the material. The permanent networks rely on covalently crosslinked networks or physical networks with sufficiently long relaxation times to remain intact within the characteristic lifetime of the temporary shape. The temporary networks and transitions rely on vitrification, melting of crystalline regions, hydrogen bonds, dipole-dipole bonds, metal complexion, charge transfer, and supramolecular bonds. Adjusting properties such as modulus and/or Tc requires changing the structure of the polymers themselves, and thus considerable effort in polymer design and synthesis.
The development of thermally sensitive SMP's has focused primarily on relatively low transition temperature (Tc<100° C.), relatively low modulus elastomeric polymers (modulus <108 Pa), such as thermoplastic polyurethanes (TPU), cross-linked polyethylene, poly(ε-caprolactone), sulfonated EPDM, and polynorbornene. Those materials are appropriate for applications such as biomedical and surgical materials, smart fabrics, and heat shrinkable tubing. Materials used in aerospace or structural components often require higher modulus (modulus >108 Pa) and switching temperatures for shape change and actuation.
Thus, while the known classes of SMP's may be suitable for their intended purposes, there nonetheless remains a need in the art for SMP's having higher modulus and switching temperatures to be able to be used in aerospace applications or structural applications.